1. Field of the Invention
This invention concerns a process for oxidizing HC1 to high-purity CL.sub.2 in a high conversion with an aqueous nitric acid-sulfuric acid mixture.
2. Description of the Prior Art
The Deacon process for oxidizing HCL to CL.sub.2 with oxygen utilizing a copper compound catalyst is old. Over the years, numerous processes for accomplishing this reaction have been developed, including some wherein the oxidizing agent is nitric acid. Among these processes is the one disclosed by Baumgartner, et al., in U.S. Pat. No. 3,152,870, and that disclosed by Jongenburger, et al., in U.S. Pat. No. 3,449,079.
The Baumgartner process is carried out by passing HCL gas countercurrently into contact with an oxidizing mixture containing 10 to 40% nitric acid, 45 to 90% sulfuric acid and as little as 5% water. The amount of water present during the subsequent regeneration is about 50 percent. The mol ratio HCL to HNO.sub.3 is about 1.0 or lower. The product gas contains about 60% chlorine and 40% nitrogen peroxide by weight. The process does not produce high-purity chlorine since the product is contaminated by about 40% nitrogen peroxide.
In Jongenburger's process, HCL is contacted with a nitric acid mixture containing at least 80% sulfuric acid, and the sulfuric acid concentration may not vary by more than 8 percent during oxidation. The amount of water present during the oxidation reaction is about 18 to 20 percent. The amount of water present during the subsequent regeneration step is about 19 percent or less. Subsequent to the oxidation, sulfuric acid containing nitrosyl sulfuric acid as spent oxidant is contacted with a mixture of air and steam to convert the nitrogen-containing species to NO and NO.sub.2. Further oxidation with air or oxygen converts nearly all of the mixture to NO.sub.2, which is converted mainly to nitric acid. The product of this process is a mixture of chlorine and HCL containing only about 77% CL.sub.2. The process does not produce high-purity chlorine since the product is contaminated by about 23% HCL.
The background section of the Jongenburger patent points out various difficulties in prior art processes. Such difficulties are said to lie in the following facts: that the product contains an excessive amount of nitrogen dioxide, that a substantial amount of hydrogen chloride is entrained in the liquid sulfuric acid reaction medium, and that high dilution of the sulfuric acid requires expensive and complicated reconcentration. Actually, Jongenburger solves these problems to a degree, primarily by introducing a new problem, a high concentration of unreacted hydrogen chloride in the chlorine product.
It has now been found that the process of this invention gives complete oxidation of HCL, no excess amount of NO.sub.2 in the chlorine product, and no excess amount of hydrogen chloride left in the sulfuric acid reaction medium. The adjustment of acid concentration is expeditiously accomplished by diverting about 5 to 10 percent of the circulating stream, stripping out nitrogen-containing species and any other volatiles, and concentrating the relatively small amount of acid to a relatively high strength.